The present invention relates to structural members and in particular to structural members which have a slotted tubular cross section. Tubular members such as pipes have been found to be useful structural members as tubular members generally provide strong resistance to bending without requiring a large cross sectional area, thereby keeping the weight per lineal foot of the tubular member low. The weight per lineal foot of a structural member is very important and should be kept as low as possible. The weight, which corresponds to the amount of material used to form the member, is the primary factor which determines the cost of the member.
In certain applications such as for fence posts and top rails the bending strength of a member about its weakest axis controls the member's design. The American Society of Testing and Materials has published a specification designated as ASTM F669 which specifies a Group IV minimum bending strength of 37,200 inch-pounds (in-lb) for corner posts; 19,600 in-lb for line posts; and 7,100 in-lb for top rails used for heavy industrial fence. The requirements of ASTM F669 Group IV are not axis oriented and apply to all axes of a structural member. A member which has a significantly larger bending strength about one axis than another axis will have a larger weight per lineal foot than what is required and will therefore be costlier than need be. Circular tubes such as pipes have therefore traditionally been used in fence construction as they have equal bending strength about all axes. While the use of a fraction of a pound of extra material in one fence post or top rail does not sound particularly costly, when the number of fence posts and top rails used in applications such as along the sides of an interstate highway which stretches for miles are considered, that fraction of an extra pound becomes very costly.
Although tubular members provide good bending strength to weight ratios, tubular members are expensive to fabricate. The member must first be rolled from a flat sheet of metal into the desired shape. The longitudinal edges of the member must then be welded together and then the weld must be ground down. These and other fabrication steps which are required to produce a tubular member add to the cost of the member.
Tubular members such as pipes which have a continuous outer wall also present problems in making a connection to the tubular member. Fastening wires have typically been used to fasten fence fabric to fence posts and top rails. The fastening wire is typically bent around the post or top rail and then each end of the wire is fastened to the fence fabric by twisting the wire around the fabric. The fastening wire must be sufficiently thin and flexible so it may be bent and twisted as needed. This subjects the fastening wire to failure due to over-twisting, corrosion or overstressing at relatively low loads. The use of fastening wires is also a very costly and time consuming process when the total number of connections between the fence fabric and the fence posts and top rails are considered.
It has therefore been found to be desireable to manufacture and use a slotted tubular structural member for fence construction and in other applications, which member has a high bending strength to weight ratio, has approximately equal bending strength about its axes, is easily fabricated and provides a strong and simple means for securely fastening objects to the member. Slotted tubular members are shown in U.S. Pat. Nos. 762,902 and 4,867,421.